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through the MN register.
All of the
operation again, is controlled by ROS
fields and micro-programming.
BASIC PROGRAMMING
SYSTEM/360 INSTRUCTIONS
•
•
•
Instructions are 1, 2 or 3 half-
words in length.
The Op code is the first byte of an
instruction and specifies
instruction length and format, gen-
eral data location, type of data,
and operation to be performed.
General Registers have a 4 bit
address and main storage has a 24
bit address.
•
The five instruction formats are
called theRR, RX, RS, SI and. SS
formats.
INSTRUCTION LENGTHS
•
System/360 instructions are 1, 2, or
3 Halfwords in length depending on
the location of data.
•
Register-to-register instructions
are 1 halfword long.
•
•
•
Storage-to-register instructions are
2 half words long.
Storage-to-storage instructions are
3 halfwords long.
Instructions must reside on halfword
boundaries (low order address bit
=
0).
Let's learn about the instruc-
tion formats of the System/360.
As
you know, instructions specify the
operation to
be
done and the loca-
tion of data.
Data may be located
in main storage, in general reg-
isters or in floating point reg-
isters.
Main storage is addressed
with a 24 bit binary address while
the general registers and floating
point registers are addressed with a
4 bit binary address.
As a result,
instructions are of different
lengths depending on the location of
data.
System/360 instructions may
be one, two, or three halfwords in
length.
When both operands or data are in
general registers or floating point
registers, only 8 binary bits are needed
for addresses.
As a result the shortest
instruction, one halfword in length, is
used.
When one of the operands is in main
storage, this short instruction cannot
be used.
Remember that main storage
requires a longer address.
For these
operations, instructions that are 2
halfwords in length must be used.
When both operands are in main stor-
age, a total of 48 bits are needed for
the addresses.
Accordingly, the longest
instruction format, 3 halfwords in
length, is used.
Because instructions are a multiple
of halfwords in length, they are consid-
ered as fixed length information as far
as
storage boundaries are concerned.
If
the address of an instruction has a low
order 1 bit, a specification exception
will occur.
OPERATION CODES
•
•
•
•
The Op Code (Operation Code) is the
first byte of each instruction.
Bits 0 and 1 specify instruction
length and data location (main stor-
age or general registers).
Bits 2 and 3 specify type of data
(fixed or variable length, decimal,
binary, or floating point).
Bits 4 through 7 specify what to do
with data.
1-35
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